The previous fix compared subPath directly against the sanitized torrent
name, which fails when RdName contains characters removed by
RemoveInvalidPathChars (e.g. ':' on Windows). Extract and normalize the
first component of subPath before comparing so both sides go through the
same sanitization. Add tests covering the duplicate-prefix stripping case.
Tests:
- [Both overloads] uses the `download.Link` to guess the filenamem if it's not present
- [Both overloads] returns null if `download.Link` is null
- [Both overloads] returns nuyll if `download.RdName` is null
- [Both overloads] works for a file in a deeply nested subdirectory
- [overload with `downloadPath` argument only] creates the correct directory
